Current institutional warewash processes involve at least 2 steps; Step 1 which is a main wash, in which the substrates are cleaned by pumping main wash solution over the substrates via nozzles. This main wash solution is obtained by dissolving main wash detergent, which can contain components such as alkalinity agents, builders, bleaches, enzymes, surfactants for defoaming or cleaning, polymers, corrosion inhibitors etc. Step 2 is a rinse step after the main wash. This is done by flowing warm or hot water, containing rinse aid solution, over the substrates, which can be followed by a hot air stream to further improve the drying process. The rinse aid typically consists of non-ionics present in an amount of 10 to 30% in water; often in combination with hydrotropes and sometimes other additives such as polymers, silicones, acids, etc.
A number of machines are used for these institutional warewash processes, such as the so called single tank, dump or multi-tank machines. Typical conditions in these institutional warewash processes are:
A. Constant temperature of main wash in a single tank and dump machines of 50-70° C.
B. Temperature of wash solution in multi-tank machine is about 40° C. in the first (prewash) tank and about 60° C. in the last wash tank.
C. High temperature of rinse solution of 80-90° C. for single tank and multi-tank machine and about 60° C. for dump machines.
D. Short total wash cycles varying from about 40 seconds to 5 minutes. The rinse cycle does not take longer than 2 minutes, and in most cases takes only between 2 and 10 seconds.
E. Wash water being re-used for many wash cycli (with exception of dump machines)
F. Volume of wash solution varying from about 5 to 10 Liter (for dump machine) to 40 Liter (for Single tank re-use machine) to 400 Liter (for multi-tank machine).
G. No carry-over of main wash solution to the final rinse solution for the so called high temperature single- and multi-tank machines. Different pumps, tubes and nozzles are used for the wash solution and rinse solution and the rinse solution is not recirculating through the wash tank during the last rinse.
H. The substrates have to be dry after the final rinse, since this is a more or less continuous batch process where the substrates are cleared away before the next batch of washed and dried substrates are coming out of the machine. These machines are used at facilities (like restaurants, hospitals, cantines) where many substrates are washed in a short period of time.
The machine and process conditions for these institutional dishwasing processes differ significantly from the conditions for domestic type of dishwash machines. Most important features of domestic dishwashing that differ from institutional ware washing are:
A. Domestic dishwash process takes about 30 minutes to 1.5 hour. The rinse cycles in these processes vary from about 5 to 40 minutes.
B. Wash solution is not re-used in the domestic dishwash process
C. Part of the wash solution is carried over into the rinse solution (e.g. via the same pump, tubes and nozzles that are used for washing and rinsing and because the rinse solution is recirculated through the wash tank during rinsing).
D. Temperature in domestic wash process is totally different; normally cold water is used for filling the machines. This water is heated up to about 60 degrees C. during the wash process.
E. Volume wash solution is about 3 to 10 Liter.
F. After the wash and rinse process there is sufficient time left for the substrates to dry further. This is facilitated by the warm conditions in the closed domestic dishwash machine.
An important recent trend in domestic dishwashing is the development of dishwash products which can be used in domestic dishwash machines without the need for a separate rinse product to be added to the final rinse solution. A key driver for this development is simplicity.
These products, often tablets, contain ingredients which facilitate the drying process. The main objective is to obtain improved visual appearance of the substrates. The most important drying-ingredients in these, so called 2-in-1 or 3-in-1 products, are polymers and non-ionics.
Crucial parameters/conditions for obtaining acceptable drying properties by this so called built-in rinse concept in domestic dishwashing machines are:
A. Carry-over of some part of the main wash solution, containing the drying ingredients, into the rinse solution. This carry-over typically takes place via the same pump, tubes and nozzles that are used for washing and rinsing and because the rinse solution is recirculated through the wash tank with dish ware during rinsing.
B. Relatively long washing time and rinsing time.
C. Relatively high area of machine surface (walls) and dish ware, on which drying components (polymers and non-ionics) will remain in the residual water that clings onto the machine parts and the dish ware. A part of the rinse components in the last rinse solution is derived from this residual water. This process of carry over of rinse components from the main wash into the rinse solution will be stimulated further when a part of the wash solution is present as foam at the end of the main wash cycle.
Despite these conditions, the drying results in domestic dishwashing machines by these tablets with built in rinse components is often inferior to drying by adding rinse component into the rinse via a separate rinse aid.
Institutional warewashing processes are characterised by very short wash and rinse cycles, i.e. by a very short contact time between the wash solution and the substrates and between the rinse solution and the substrates. In addition, in institutional high temperature single- and multi-tank machines there is no carry-over of the wash solution via the pump, tubes and nozzles of the machine and no carry-over by adsorption and subsequent desorption via the machine walls (since the rinse solution is not recirculated in the wash tank). Therefore, the concept of built-in rinse components is not expected to work in institutional warewashing processes. Furthermore, reduced drying times are much more important for institutional warewashing processes than for domestic dishwashing, where emphasis is on visual appearance.
Therefore, all proper warewashing processes in institutional warewashing machines require the need for rinse components to be present in the final rinse solution, which are introduced by dosing a separate rinse aid in this rinse solution.
One attempt to develop a main wash detergent product for institutional warewashing machines with a built-in rinse component is described in U.S. Pat. No. RE 38,262. In this patent high levels of non-ionics (20-40%) are needed to obtain visual drying benefits when not adding rinse agent to the rinse water. This amount of rinse agent ensures that the detergent composition contains sufficient source of alkalinity and other components to adequately clean the dishes while leaving a sufficient concentration of a rinse agent residue on the layer and the internal structures of the machine including rack and ware, spray arms, walls, etc. to promote rinsing or sheeting in the potable water rinse cycle. In particular, it has been found in U.S. Pat. No. RE 38,262 that the concentration of the nonionic sheeting agent in the aqueous rinse commonly is about 20 to 40 parts by weight or more per million parts of the aqueous rinse if the alkaline detergent material contains greater than about 25 wt % of the nonionic sheeting agent.
The process described in the examples of U.S. Pat. No. RE 38,262 has high similarity to the carry over effects which lead to built in rinse effects in domestic dishwashing processes. Crucial is that nonionics are dissolved in the rinse solution and so lead to improved visual drying effects. The level of carry over is determined by the type of warewashing machine and for that reason the so called dump low temp machines are preferred for this process.
These high levels of nonionics are very difficult to incorporate in a main wash detergent without sacrificing physical properties like flow and stability and will lead to high costs.